Optical elements or components are omnipresent in devices, systems or arrangements where light needs to be directed, expanded, focussed, collimated or otherwise transformed or affected. Optical elements can for example be embodied by lenses, mirrors, Diffractive Optical Elements (DOE), assemblies thereof, or the like.
In a typical optical system, most or all optical elements usually need to be precisely positioned and aligned for them to perform the correct optical function. This positioning and alignment typically involve securing the optical element in a holder or mount of some sort. Proper alignment of an optical element with respect to the holder is a delicate operation which generally requires tight manufacturing tolerances and careful handling.
Barrels are well known types of mechanical holders for optical elements. Barrels typically define a cylindrical cavity in which is mounted one or more optical elements. By way of example, a lens is a type of optical element that is often mounted in barrels. A lens, in its simplest expression, typically consists of a construction of an optical material having opposite surfaces, at least one of which being partially spherical, either convex or concave. Compound lenses, made of several different lenses arranged in a cascade, are also well known in the art.
FIG. 1 (PRIOR ART) shows a biconvex lens having first and second convex surfaces S1 and S2, illustrating the geometrical parameters characterising the lens. Each surface S1 and S2 has a corresponding center of curvature C1 and C2, which is defined as a point lying at a distance from the surface corresponding to the radius of curvature R1 and R2 of the surface, at a normal vector. In other terms, the center of curvature C1 or C2 may be imagined as the center of a sphere SP1 or SP2 of which the corresponding surface S1 or S2 of the lens would be a portion. The optical axis A of the lens can be defined as the line joining the centers of curvature C1 and C2 of both opposite surfaces S1 and S2 of the lens.
A lens generally needs to be centered with a precision that can be of the order of a few micrometers, taking under consideration all the parameters defined above. Opto-mechanical assemblies in which lenses are mounted and precisely centered are known in the art. Referring to FIG. 2 (PRIOR ART), there is shown a typical assembly 20 including a lens 22, a barrel 24 and a retaining ring 26. The lens 22 is mounted in the barrel 24 with the periphery of one of its surfaces S1 in contact with a lens seat 28. The retaining ring 26 is typically threaded within the barrel 24 and abuts on the surface S2 of the lens 22 opposite to the lens seat 28, thus securing the lens 22 in the assembly 20. It is well known in the art that the lens is centered when both centers of curvature C1 and C2 lie on the center axis B of the lens barrel 24.
The technique consisting in inserting a lens in a lens barrel and then securing the lens with a threaded ring is generally referred to as the “drop-in” lens technique. The centering precision obtained from this technique first depends on the minimum allowable radial gap between the lens and the barrel. Thermal effects caused by the mismatch of the respective coefficients of thermal dilatation of the lens and of the barrel materials also have an impact on the centering of the lens. Manufacturing tolerances on dimensions of the assembly components such as the diameter of the lens, the diameter of the barrel cavity and the thickness difference along the edge of the lens also affect the quality of the centering. The greater the required precision on the centering of the lens, the greater the manufacturing costs of both lens and barrel.
The main advantages of the drop-in technique are that the assembly time can be very short and that the lenses are removable. Low cost drop-in however has the drawback of a loss in centering precision. If more precision is required, the drop-in method may not be suitable and an active alignment is typically required. In this centering method, the lens is first positioned inside the cavity and its decentering relative to the reference axis of the barrel is measured. The lens is then moved to reduce the centering error. These steps can be repeated several times until the decentering of the lens complies with the centering requirements. Once centered, the lens is fixed in place with adhesive or other means. This method provides a very high level of centering accuracy, but requires expensive equipment while being time-consuming.
While the discussion above concerns mainly lenses, other types of optical elements can be mounted in a barrel using a retaining ring, and such elements are confronted with the same issues discussed above.
There therefore remains a need for an approach for mounting an optical element in a barrel which alleviates at least some of the drawbacks of known techniques.